A technique of the inverter circuit which is driven at a high frequency is applied to various equipment such as an induction heating cooking device which inductively heats a metallic pan using a high-frequency magnetic field, an industrial induction heating apparatus aiming at dissolution or quenching of metal, and an X-ray imaging apparatus and an X-ray CT (Computer Tomography) apparatus for a medical/industrial use to acquire an X-ray perspective image and a tomographic image of a test subject.
Typically, an inductor or an X-ray tube is considered as a load of the inverter circuit in those fields as described above, and its impedance may be largely different or change according to setup conditions of voltage and current, and temperature. Therefore, fluctuations occur in the resonance frequency of the load circuit. For example, as for the X-ray tube load, equivalent resistance may fluctuate drastically in the range of several tens of kΩ to several tens of MΩ according to a setup condition of X-ray output. The inverter circuit needs to control an operating frequency by tracking the load fluctuation, and simultaneously, needs to control the output according to the setup conditions of voltage and current.
Conventionally, PLL control has been employed against the load fluctuation, and as for the output control, voltage of a converter provided in the former stage of the inverter circuit is controlled, thereby simultaneously achieving both functions above. However, in this conventional apparatus, the control system is complicated and large in size, and thus the patent document 1 discloses a technique for implementing both functions by a single inverter circuit. In order to make the inverter operating frequency coincide with the change of resonance frequency caused by the load fluctuation, the technique disclosed by the patent document 1 detects current passing through two semiconductor switches or diodes inversely connected in parallel to the semiconductor switches, which are incorporated in the inverter circuit, and changes the inverter frequency in such a manner that current flowing periods become equal. In addition, in order to control the output power to be a desired value, a phase shift angle is controlled at which each of the semiconductor switches of the inverter circuit operates. With the configuration as described above, a single inverter achieves both the control for tracking the resonance frequency change due to the load fluctuation, and the control of power.